1. Field of the Invention
The present invention relates generally to a turbine airfoil with film cooling holes, and more specifically to an electrode for forming the film cooling holes.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
A gas turbine engine includes a turbine section with multiple rows of stator vanes and rotor blades that interact with a hot gas flow to extract mechanical energy used to drive a bypass fan in the case of an aero engine or drive an electric generator in the case of an industrial gas turbine (IGT) engine. The efficiency of the engine can be increased by passing a higher temperature gas flow into the turbine. However, the turbine inlet temperature is limited to the material properties of the airfoils and the cooling effectiveness of these airfoils. The first stage airfoils are exposed to the highest temperature gas flow. Thus, it is the first stage airfoils that typically limited the turbine inlet temperature.
An improvement in the cooling effectiveness of an airfoil can include better convective cooling, impingement cooling or film cooling or a combination of these three types of cooling. The key to better cooling effectiveness is controlling the metal temperature of the airfoil. The higher the metal temperature, the more likely hot spots will occur that produce erosion, or the higher thermal stress will result in excessive creep damage or other stress induced problems that severely limit the part life. In an IGT, part life is critical since these engines operate continuously for period in excess of 48,000 hours before being shut down for service and repair.
In the prior art, a film cooling flow channel includes a straight circular entrance region of constant diameter that forms a flow metering portion for the film hole. Following the metering section is a single diffusion section that opens onto the airfoil surface. The conical diffusion section is used to reduce the cooling air flow momentum or exit velocity. Normally, an expansion area ratio of 2 to 6 times the metering section area is used in the prior art airfoil film cooling hole design. This type of film cooling hole construction can be found in most of the current diffusion holes which are widely used in the current film cooling design for airfoils. The shaped film hole includes a 10 degree downstream expansion (in the direction of the hot gas flow over the film hole) and a 10 degree spanwise expansion in the radial outward and radial inward directions. No expansion is used in the upstream direction. U.S. Pat. No. 4,684,323 issued to Field on Aug. 4, 1987 and entitled FILM COOLING PASSAGES WITH CURVED CORNERS discloses this type of film cooling hole. An expansion in the upstream direction will entrain the hot gas into the film cooling hole at the exit plane where the film cooling hole merges with the airfoil surface. As a result of the hot gas entrainment, shear mixing with the ejecting cooling air will result which degrades the film effectiveness level of the film cooling air.
Film cooling holes are formed using an electrode that has the shape of the film cooling hole which is used to burn or melt away (using the EDM or electrode discharge machining process) the airfoil material and leave the film cooling hole with the desired shape. U.S. Pat. No. 4,650,949 issued to Field on Mar. 17, 1987 and entitled ELECTRODE FOR ELECTRICAL DISCHARGE MACHINING FILM COOLING PASSAGES IN AN AIRFOIL discloses the sheet metal form of an electrode that is used in the early periods to produce the 10×10×10 shaped diffusion holes by the electrode discharge machining (EDM) process.
The sheet metal electrode has evolved into a three dimensional solid electrode. The solid electrode produces the prior art 10×10×10 diffusion shaped film hole used in the current turbine airfoil cooling holes. The foot print is in a trapezoid shape with four straight sidewalls. The central axis of the metering hole is tangent with the upper sidewall of the trapezoid while the sidewalls are perpendicular to the film hole upstream and downstream walls.
A three dimensional diffusion hole in a compound angled orientation to the mainstream gas flow with a variety of expansion angles are also used in the airfoil cooling design for further enhancement of the film cooling capability. FIG. 3 shows an electrode for forming this compound angle shaped diffusion hole with zero expansion angle on the spanwise radial direction and 10 degrees expansion in the spanwise downstream and streamwise downstream directions. FIG. 4 shows a side view of the electrode in FIG. 3. The prior art three dimensional diffusion shaped film hole is constructed with both sidewalls perpendicular to the film hole upstream and downstream walls. A further improvement over the prior art traditional standard 3D diffusion can be achieved by convergent or divergent sidewalls.